The present invention relates to implantable electrical stimulation devices and more particularly to the fixation of microstimulators included in Peripheral Nerve Stimulation (PNS) systems. Such microstimulators may migrate over time and become ineffective.
Implantable electrical stimulation devices have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, in recent investigations Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems such as the Freehand system by NeuroControl (Cleveland, Ohio) have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
Known implantable electrical stimulation systems typically consist of a System Control Unit (SCU), which contains a power source and stimulation electronics, and electrodes connected to the SCU by leads. A number of SCU systems have the advantage of having fixation devices for the electrodes so that the electrodes remain proximal, or even attached, to their target sites of stimulation. For example, some pacemaker electrode leads have tines or fins that act as barbs to hook into the tissue, thereby anchoring the electrodes in place. As another example, the electrode used in the Neuro Cybernetic Prosthesis (NCP®) manufactured by Cyberonics (Houston, Tex.) is a helical electrode that is wound around the vagus nerve in order keep the electrode attached to the stimulation target. Finally, several companies and research institutions, such as Neuro Stream Technologies, Inc. (Anmore, British Columbia, Canada), are investigating cuff electrodes which wrap around the nerve like a cuff, thereby fixing an electrode(s) proximal to a nerve.
While leaded system have been successful in many applications, several potential disadvantages accompany their use. The implantation procedure may be rather difficult and time-consuming, as the electrodes and the SCU must usually be implanted in separate areas, and the lead must be tunneled through body tissue to connect the electrodes to the SCU. Also, the leads are typically thin and rather long and are thus prone to mechanical damage over time. Additionally, many conventional systems typically consist of a relatively large SCU, which can have a negative cosmetic appearance if positioned subcutaneously.
A microminiature electrical stimulator, a microstimulator, is described in U.S. Pat. No. 5,193,539 issued May 16, 1993 for “Implantable Microstimulator.” A method for manufacturing the microstimulator is described in U.S. Pat. No. 5,193,540 issued May 16, 1993 for “Structure and Method of Manufacturing of an Implantable Microstimulator.” Further teaching is included in U.S. Pat. No. 5,324,316 issued Jun. 28, 1994 for “Implantable Microstimulator.” WO 97/29802 published Aug. 21, 1997 for “Improved Implantable Microstimulator and System Employing Same,” describes a microstimulator coated with a biocompatible polymeric providing increased strength and therapeutic effects. WO 98/43700 published Oct. 8, 1998 for “System of Implantable Devices for Monitoring And/Or Affecting Body Parameters,” describes a system including microdevices, for monitoring and affecting the parameters of a patient's body. WO 98/43701 published Oct. 8, 1998 for “System of Implantable Devices for Monitoring And/Or Affecting Body Parameters,” describes a system including a System Control Unit (SCU) and one or more microdevices, for monitoring and affecting the parameters of a patient's body. And, WO 00/56394 published Sep. 28, 2000 for “Ceramic Case Assembly For a Microstimulator” describes a ceramic case for a microstimulator, which case is transparent to an alternating electromagnetic field and offers strength superior to previous case materials. The '539, '540, and '316 patents, and the '802, '700, '701, and '394 PCT publications are incorporated herein by reference.
The microstimulator was developed to solve a number of issues related to implantable stimulation devices, including overcoming some of the disadvantages of a leaded system. The SCU and the electrodes of the microstimulator have been combined into a single microminiature package, thus eliminating the need for a lead. A typical embodiment of a microstimulator is housed in a cylindrical case that is about 3 mm in diameter and between 2 and 3 cm in length. This form factor allows a microstimulator to be implanted with relative ease and rapidity, e.g., by injection or via endoscopic or laparoscopic techniques. Known microstimulators are held in place by the tissue that surrounds the case and do not include means of fixation or stabilization other than certain coatings which may be applied to the surface of the case to allow greater adhesion to the surrounding tissue.
In some PNS applications, the microstimulator is surrounded by soft tissue and is not embedded in, or located very close to, large muscles or other structures that experience significant motion or varying pressure. In such applications, additional means for fixation of the microstimulator is not required. However, in other applications, fixation of the microstimulator may be necessary. For example, some peripheral nerves are located immediately next to, or are embedded within, muscles. Contraction of the muscles may cause the microstimulator to migrate significantly over time. Additionally, the motion of limbs may exert unequal pressures on the microstimulator at almost any location within an extremity, which may also result in migration, especially along the pathway created during implantation.
As yet another example, some nerves are surround by firm but lubricious tissue, and such tissue may not provide sufficient fixation of the microstimulator. The vagus nerve runs through the carotid sheath in the neck, which is surrounded by a number of muscles. The firm, lubricious carotid sheath is likely to offer little adhesion to the microstimulator, and actuation of the surrounding neck muscles may cause significant migration of the microstimulator, especially during the period immediately following implantation.
What is needed is a means for fixation for a microdevice when the cooperation of the case with surrounding tissue is not sufficient to prevent migration.